synthesis and characterization of polyaniline–hydrotalcite
TRANSCRIPT
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PAPER
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Synthesis and ch
aInstitute of Chemical Technology, Vietnam
TL29, Thanh Loc Ward, District 12, Ho Ch
vast.vnbInstitute of Tropical Technology, Vietnam
Hoang Quoc Viet, Cau Giay, Hanoi, VietnamcGraduate University of Science and Techn
Hanoi, Vietnam
Cite this: RSC Adv., 2021, 11, 31572
Received 17th June 2021Accepted 6th September 2021
DOI: 10.1039/d1ra04683g
rsc.li/rsc-advances
31572 | RSC Adv., 2021, 11, 31572–315
aracterization of polyaniline–hydrotalcite–graphene oxide composite andapplication in polyurethane coating
Boi An Tran, *ac Huynh Thanh Linh Duong,a Thi Xuan Hang Tobc
and Thanh Thao Phanac
In this paper, a composite from polyaniline and graphene oxide-hydrotalcite hybrid (PAN–HG) was
fabricated by direct polymerization of aniline using ammonium persulphate as an oxidant in the presence
of a HG hybrid. The structure and morphological properties of synthesized PAN–HG composites were
characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman spectra,
and scanning electron microscopy (SEM) techniques. The electrochemical properties of the composite
particles were also analyzed by potentiodynamic polarization curves to evaluate the corrosion
inhabitation. The results were calculated by Tafel fitting and showed that the effective corrosion
protection values were 73.11%, 88.46%, and 95.49%, corresponding to HG, 1PAN–HG, and 2PAN–HG.
The influence of PAN–HG on the corrosion protection of the polyurethane coating applied on the CT3
steel was investigated. As a result, the PU containing 0.5% of 2PAN–HG showed the most effective
protection of the CT3 steel substrate. The RC of the coating was about 1.61 � 107 U cm2, and after
immersion for 30 days, the RC value was 0.17 � 106 U cm2. From all the analyzed results, PAN–HG has
enhanced the corrosion protection and a complicated protection mechanism was also concluded and
explained.
Introduction
In recent years, metal corrosion causes damage in variouselds, such as in transportation (e.g., automobiles, aircras,ships), infrastructure (e.g., pipelines, bridges, buildings) andindustrial manufacturing (e.g., oil-shore, production machine).Corrosion inhibition is the most effective solution, and hasbeen studied continuously with various materials and agents.Many compounds are toxic or not entirely harmless/environmentally-friendly. Therefore, new anticorrosion agentsare necessary, which have the same efficient (but less harmful)heavy metals (e.g., Pb, Cr(VI), Cd, Ni).1–3 Green corrosion inhib-itors are worthy candidates because they have high efficiency,and are less or not harmful to the environment or humanbeings.4–8 Green corrosion inhibitors are biodegradable andnaturally-occurring substances. According to their chemicalnature, they can be classied into two broad categories: organicgreen inhibitors and inorganic green inhibitors. Some organic
Academy of Science and Technology, 1A
i Minh City, Vietnam. E-mail: tban@ict.
Academy of Science and Technology, 18
ology, 18 Hoang Quoc Viet, Cau Giay,
82
green inhibitors are amino acids, polymers and biopolymers,surfactants, plant extracts, chemical medicines and ionicliquids. Inorganic green inhibitors are rare earth elements andclays or minerals. Here, we are interested in the combination ofboth inorganic and organic green inhibitors to improve theanticorrosion efficiency. Specically, polyaniline was used as anorganic green inhibitor and the hydrotalcite–graphene oxidehybrid was used as an inorganic green inhibitor.
Hydrotalcite (HT), a kind of double-layered hydroxide, is oneof the inorganic mineral groups. HT is evaluated as an envi-ronmentally friendly material, and also a green inhibitor. HT isused as a layer coated on metal surfaces,4–6 or as an additive inthe organic coating. HT has been modied to improve polarity,as well as to increase dispersibility in the coating to achievea uniform coating. Moreover, the modied HTs have highadsorption capacity, which helps to intercalate the moreinhibitor molecules to their structure. When used, the inhibi-tor's molecules will be released into the coating to inhibitcorrosion.7–11 The vanadate ion has been intercalated into thedouble-layered structure of HT via an anion exchange mecha-nism, and HT–V was then used in an organic coating to protectthe aluminium alloy.16 The inhibitive effect of HT–V can becompared with highly effective chromate inhibitors. The resultof the salt spray test on Al alloy coated epoxy-containing HT–Vshows that the inhibitive efficiency was maintained aer 1000hours exposure in salt fog.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Fig. 1 FT-IR spectra of PAN (a), 2PAN–HG (b), 1PAN–HG (c) and HG(d).
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To enhance the inhibitory efficiency, HT has also beenstudied in fabricating a hybrid material or nanocomposite withgraphene oxide (GO) because of the high barrier behavior ofgraphene oxide.12–16 The preparation method of the nano-composite from graphene oxide–hydrotalcite (GO–HT) has beeninvestigated for fabrication,21 wherein the membrane was usedas nanoltration for water desalination,17,18 and co-precipitatingHT in the presence of GO. It has been used as a corrosioninhibitor,13,19,20 for gas adsorption,21–23 water pollution treat-ment,24,25 catalyst26–28,32,33 and energy applications.29–31 An inor-ganic lm of GO–HT was successfully coated on the aluminiumalloy 6N01 surface, and showed the perfect barrier against thepermeation of H2O, O2 and Cl�.17 Similar to HT, GO–HT can bemodied with appropriate anion and organic derivatives toimprove the anticorrosion efficiency. GO–HT was also used asan anticorrosion ller in organic coatings, such as polyvinylalcohol, PMMA, and acrylic lm.20,37–39 These research studiesall proposed the same anticorrosion mechanism of GO–HT inorganic coating, which is the enhancement of the barrierbehavior.
Polyaniline (PAN) is a conducting polymer, and it hasrecently gained great efficiency in anticorrosion applicationbecause PAN has a complex anti-corrosion mechanism,including anodic protection,40,41 cathodic protection,42,63,64
controllable inhibitor release,43–49 and barrier behavior.49 PANwas widely used in various resins, such as epoxy, alkyd, acrylic,and efficiency.49–51 When doping with camphor sulphonate,phenyl phosphonate and iodine, the doped PAN gives a doubleprotection effect.49,58 At rst, the redox reaction that occursbetween Fe and PAN leads to the reduction of PAN and therelease of the doped anion. Then, the combination of the Fecation and reduced PAN can give a passively complex layer toprevent the penetration of ions. Moreover, PAN was studied tofabricate a composite with either organic compounds, such assulfonated chitosan,52 2-mercaptobenzothiazole,53 benzoate,69
nylon 66,54 and palm oil;55 or inorganic clays, such as magnetiteclay, aluminium oxide and gamma-alumina.24,34,56 All of thosestudies show that composites from PAN give good anti-corrosion performance in both HCl and NaCl environments.
In general, the most common anti-corrosion mechanism ofGO–HT is the high barrier ability, while PAN supports bothanode and cathode protection mechanisms. The hybrid mate-rial from GO–HT, as well as the nanocomposite/composite fromPANwere widely studied for use as either corrosion inhibitors orused in the protective organic coating. However, there have notbeen any reports about the combination of PAN and GO–HT toproduce a composite material that obtains an enhanced matrixmechanism of anti-corrosion.
Owing to the above advantages of GO, HT, and PAN, in thispaper, we focused on a matrix from GO, HT, and PAN fora nanocomposite. HT was prepared using the co-precipitationmethod in the presence of GO to form a HG hybrid. Then,aniline was polymerized in the presence of HG via oxidationpolymerization to produce the PAN–HG nanocomposite. Theinuence of PAN–HG on the anti-corrosion on carbon steel (CT3steel) will also be investigated using the potentiodynamicpolarization method, and then analyzed with a Tafel t. PAN–
© 2021 The Author(s). Published by the Royal Society of Chemistry
HG was fabricated with various ratios of mPAN/mHG to investi-gate the inuence of PAN on the anti-corrosion effect. In thispaper, we fabricated 1PAN–HG and 2PAN–HG, which haveratios of mPAN/mHG ¼ 1/1 and 2/1, respectively.
Results and discussionCharacterization of composite particles polyaniline–HG(PAN–HG)
Fourier transform IR (FT-IR). Chemical characterizations ofHG, PAN, 1PAN–HG, and 2PAN–HG are shown in the FT-IRspectra (Fig. 1). GO was characterized by strong absorptions at1716, 1616, 1384, and 1053 cm�1, which were attributed to –C]O in the –COOH group, –C]C– in the benzene ring, –C–OH and–C–O–C–, respectively. In the FT-IR spectra of HG, there areblueshis from 1716 cm�1 to 1562 cm�1 and from 1616 cm�1 to1357 cm�1 because of the increase of bond length due to theinteraction of GO and HT. Moreover, the bands at 767 and609 cm�1 refer to the Zn–O and Al–O vibrations, which repre-sent the basic lattice of HT.57
PAN presented with strong absorptions at 3443, 2939, 1515,1481, 1288, 1130, and 875 cm�1, which was evidence of theemeraldine form of PAN.29,30 The band at around 3443 cm�1 wasassigned to N–H stretching, and the band at around 2939 cm�1
was assigned to the aromatic C–H stretching. The characteristicstretching of the quinoid ring structure, and the band at1481 cm�1 was attributed to both C]C and C–N of the benze-noid ring structure. All of these absorption peaks were charac-terized for the in-plane bending vibration. The band at around1288 cm�1 indicated the vibration of the C–N stretching of thebenzenoid ring, and the one at around 875 cm�1 was the C–Hout-of-plane bending vibration parade-substituted benzene,indicating polyaniline formation.59 In addition, the strong bandat 1130 cm�1 was assigned to the C–H bending attached to thequinoid ring.
The PAN–HG composite represented the same characteristicbands as the pure PAN, and the corresponding bands of thePAN–HG composite were shied to higher wavenumbers, whichcan be called a red-shi. This indicated that the 2D HT nano-sheets affected the band positions in the FT-IR spectrum. The
RSC Adv., 2021, 11, 31572–31582 | 31573
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C–N stretching band of quinoid at about 1515 cm�1 shied to1579 cm�1, and the C]N stretching band at about 1481 cm�1
shied to 1490 cm�1. In contrast, the C–N stretching band ofbenzenoid at about 1288 cm�1 shied to a lower wavelengthband. These shis in the spectrum indicated that the 2D HTnanosheets were attached to the PAN surface. It was due to thestretching of benzenoid and quinoid, which made the oscilla-tion energy increase. On the other hand, the band at about3236 cm�1 was assigned to the N–H hydrogen bond presentedon the HG surface, which also conrmed the successfulcomposite from GH with PAN.
X-ray diffraction. The crystal structures of HT, GO and HGwere characterized by XRD (Fig. 2). The XRD pattern of GOexhibits a sharpened peak at 2q ¼ 10.28�, indicating the (0 0 1)plane. It also has a broad peak at a narrow angle of diffraction,which is from an amorphous structure (2q ¼ 2–5�).
The composition of the material and the structure weredetermined by the XRD patterns, which are shown in Fig. 2. Asthe results show, the characteristic diffraction peaks of HG arethe same as the diffraction peaks of HT and appear at 2q of11.70� (003), 23.56� (006), 34.58� (012), 39.56� (015), 46.78�
(018), 60.26� (110), and 61.64� (113), which are attributed to theformation of nano-plates and the hydroxide layered structurecombined with graphene oxide sheets. In the HT XRD pattern,there is a small amount of boehmite, which caused a decreasein the material crystallinity. Meanwhile, weak peaks of ZnO andbayerite were identied in the HG XRD pattern. The reason ofthis can come from the reduction reaction of Zn2+ with func-tional groups on the GO surface. This result shows that the HTcrystals are stable and growing on the GO plates. At the sametime, HT linked on the surface of larger GO plates will havediffraction coinciding with the surface diffraction (100) by GO.
d ¼ (n � l)/(2 � sin q) (1)
The Raman diagram of GO (Fig. 3) shows that there are 2sharp peaks with strong intensity at 1346 cm�1, 1588 cm�1, and1 peak with very weak intensity at 2700 cm�1, which are
Fig. 2 XRD pattern of GO (a), G–HT (b) and HT (c).
31574 | RSC Adv., 2021, 11, 31572–31582
characterized as the D peak, G peak, and 2D peak, respectively.The G peak is weaker than the D peak because the oxidationreaction attaching the polar functional groups on the GOsurface from the initial bonds in the strong graphite latticeincreases the number of C–sp3 bonds compared to C–sp2 bonds.The relative intensity ratio of the D peak to the G peak is ID/IG ¼1.04, indicating that the graphite blocks have been stripped andoxidized to GO. The Raman diagram of the HG sample clearlyshows the D peak and G peak in the range of 1596–1600 cm�1,respectively. The relative intensity ratio of these two peaksincreased, specically ID/IG ¼ 1.30, as a result of the reductionin the sp2 bond size in the hexagonal structure of GO, structuraldefects in the GO network, as well as the reduction of O func-tional groups in the GO structure.35,36 In addition, HG materialspose the full Raman scattering characteristics of HT materialsin the region of 582 cm�1 and 1092–1116 cm�1. These Ramanscattering analysis results are in agreement with the publishedresults of Xiaohu Luo17 and Ningning Hong.20
The XRD pattern of PAN shows that it has two broaddiffraction peaks at 2q¼ 20.28� and 26.84�, which are attributedto the periodicities parallel and perpendicular to the polymerchain, respectively. The XRD patterns of both 1PAN/HG and2PAN/HG have broad diffraction peaks at around 20.28� and26.84�, respectively, corresponding to the pure PAN. In thesediffraction peaks, 2PAN/HG with the higher ratio of mPAN/mHG
obtained stronger diffraction than 1PAN/HG. In addition, both1PAN/HG and 2PAN/HG have diffractions at 36.34�, 37.85� and58.79� corresponding to the le shi of the diffraction of HG atthe (009), (015) and (110) planes, respectively. The XRD resultsclearly show that the PAN particles are not intercalated in theinterlayer of HG, and the layered double hydroxide structure ofHG is maintained (Fig. 4).
SEM images
The morphology of the material was observed by SEM images inFig. 5. The morphology of HG still has GO plates, but with HT
Fig. 3 Raman spectra of graphen oxide (GO) and hydrotalcite–GOhybrid (HG).
© 2021 The Author(s). Published by the Royal Society of Chemistry
Fig. 4 XRD patterns of HG (a), 1PAN–HG (b), 2PAN–HG (c) and PAN(d).
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crystals distributed on the surface. This helps the GO plates not befolded by the physical bonding. PANhas needle shape crystals dueto the preparation method, which using tartaric acid as anorientation agent for crystal development. This is consistent withthe publication of C. Muthuselvi et al.60 The SEM image of PAN–HG shows that the nanoparticles of HG were attached to the PANcrystal. This result is completely consistent with the results of thestructural analysis by the signicantly reduced X-ray diffraction.
Corrosion protection composite
The passivation of CT3 steel in an electrochemical system with/without composite particles was examined using the potentio-dynamic technique. The behavior of the potentiodynamicpolarization curves for the CT3 steel surface (working electrode)indicates whether the CT3 steel surface is cathodically protected(sacricial anode principle) or anodically protected (passivationprinciple).61–66 The CT3 steel surface was exposed to a 3.5% NaClaqueous solution and a series of 3.5% NaCl aqueous solutionscontaining 1% (wt/wt) GO, PAN, HG, 1PAN–HG, and 2PAN–HG,and were labeled GO–sol, PAN–sol, HG–sol, 1PAN–HG–sol, and2PAN–HG–sol, respectively. All of the solutions in the electro-chemical cells were magnetically stirred for 2 hours. Fig. 6
Fig. 5 SEM images of HG (a), PAN (b), 1PAN-HG (c) and 2PAN-HG (d).
© 2021 The Author(s). Published by the Royal Society of Chemistry
shows the potentiodynamic polarization curves and the kineticparameters obtained aer Tafel tting using EC-LAB analystversion 10.36, and is also presented in Table 1.
Fig. 6a shows the Tafel plots of CT3 steel in 3.5% NaCl, GO–sol, and HG–sol. Fig. 6b shows the Tafel plots of CT3 steel inHG–sol, PAN–sol, 1PAN–HG–sol and 2PAN–HG–sol. The Ecorr ofCT3 steel in bare 3.5% NaCl solution was �802.24 mV. Thepresence of GO, HG, and composite particles caused the shi ofEcorr to more positive values, which are �740.24 mV,�495.03 mV, �682.64 mV, �406.97 mV, and the highest is�381.99 mV, corresponding to GO–sol, HG–sol, PAN–sol,1PAN–HG–sol, and 2PAN–HG–sol, respectively.
The corrosion current density (Icorr) is directly proportionalto the corrosion rate at the point of intersection of the anodicand cathodic curves. In GO–sol, Icorr decreased to 6.24 mA cm�2
compared to NaCl–sol, which is 27.73 mA cm�2. Thus, thecorrosion protection reached 77.50%. The Icorr of HG–sol andPAN–sol are 7.77 and 6.32, respectively, which are lower thanthat for NaCl–sol, but higher than that of GO–sol due to thelower barrier ability of HG and electrical conductivity of PAN. Asa result, the corrosion protections are 71.11% and 77.21%. TheIcorr values of 1PAN–HG–sol and 2PAN–HG–sol have signi-cantly decreased to 0.92 and 0.36, respectively, corresponding tothe corrosion protection of 88.46% and 95.49%. The presence ofPAN in the structure of the composite PAN–HG helps toenhance the protection on the CT3 steel surface due to thecomplicated protection mechanism of PAN. Those mechanisms
Fig. 6 Potentiodynamic polarization curves of CT3 steel in 3.5% NaClsolution without/with GO, HG, PAN, 1PAN–HG and 2PAN–HG.
RSC Adv., 2021, 11, 31572–31582 | 31575
Table 1 Tafel fitting results from the potentiopolarization curves
Sample Ecorr, mV vs. Ag/AgCl/KCl Icorr, mA cm�2 ba, mV dec�1 �bc, mV dec�1 Rcorr, mm year�1 h, (%)
Bare CT3 �802.24 27.73 123.4 240.1 52.33 —GO �740.24 6.24 179.5 106.6 11.78 77.50HG �495.03 7.77 60.6 165.0 14.64 73.11PAN �682.64 6.32 123.0 80.0 10.33 77.211PAN–HG �406.97 3.20 160.5 189.4 0.02 88.462PAN–HG �381.99 1.25 273.1 318.1 0.84 95.49
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are anodic protection,45–50 cathodic protection,50–53 controlledinhibitor release,54–56 and barrier protection.72
From the results of corrosion protection, both 1PAN–HG and2PAN–HG have the same corrosion protection. So, both of themwill be combined in the PU coating, and investigated for anti-corrosion organic coating.
Characterization of anti-corrosion coatings
The PU coatings containing HG, 1PAN–HG, and 2PAN–HG werefabricated on a well prepared CT3 steel surface and labeledPU(HG), PU(1PAN–HG), and PU(2PAN–HG), respectively.
The microstructure and dispersion of composite particles inthe PU coating were observed by SEM images in Fig. 7. The SEMimages show that both HG, 1PAN–HG, and 2PAN–HG are welldispersed in the PU coating, and that will help increase theprotective effect of the PU coating.
On the other hand, the presence of PAN causes the increasein the contact angle of the PU coating surface. This helps inincreasing the barrier property of the coating, and constrainsthe diffusion of water into the coating. The diagram in Fig. 8shows the contact angle dropwise on the surface of PU(HG),PU(1PAN–HG), and PU(2PAN–HG), which are 80.8�, 103.9�, and107.1�. Therefore, PU(2PAN–HG) was predicted to be the mostanti-corrosion effective, and the electrochemical analysisresults will be discussed.
In this research, the EIS results obtained from the PU coatingcontaining HG, 1PAN–HG, and 2PAN–HG aer 1, 5, 10, and 30days immersion in 3.5% NaCl solution are shown in Fig. 10.
Fig. 7 Microstructure of PU (a), PU(HG) (b), PU(1PAN–HG) (c), andPU(2PAN–HG) (d) observed from SEM images.
31576 | RSC Adv., 2021, 11, 31572–31582
The changes in the protection performance of the coatingscan also be illustrated by Nyquist plots. Aer 1 day immersion,the Nyquist plots of PU(2PAN–HG) show a one-time constantwith only one arc, while Nyquist plots of PU(HG) and PU(1PAN–HG) have more than one arc. The corrosive medium did notreach the metal surface because of the high barrier of PU(2PAN–HG), which was proved by the contact angle of the dropwisecoating surface.73 Aer 5 days, 10 days, and 30 days, all Nyquistplots of PU(HG), PU(1PAN–HG), and PU(2PAN–HG) have morethan one arc because the corrosive medium diffused throughthe coating and reached the metal surface, so that corrosion hasoccurred. The electrical equivalent circuit shown in Fig. 9 wasemployed to t the EIS data using the soware EC-LAB version10.36. RS, RC, and RCT represent the electrolyte resistance, thecoating resistance, and the charge transfer resistance, respec-tively. The CPEC and CPEdl are related to the constant phaseelement of the coating capacitance and double layer due to thedispersion effect.
Nyquist plots illustrate the changes in the protectionperformance of the coatings. As the results from Table 2 show,the RC of PU(HG) has slightly increased in the short immersionperiod. The RC aer 1 day immersion is 3.50 � 106 U cm2, andthen it increases to 3.96 � 106 U cm2 aer 5 days immersion. Itis due to the anticorrosion mechanism of HG, which isexplained as the ion exchangeable hydrotalcite to trap Cl�, aswell as electron trapping of graphene to reduce the erosion ofthe medium.74 However, Cl� ion exchange and electron trap-ping of HG have limited capability. Therefore, the RC of PU(HG)decreased to 1.91 � 103 U cm2, and then 0.40 � 103 U cm2 aer20 days and 30 days, respectively.
Fig. 8 Contact angle of water dropwise on the PU(HG), PU(1PAN–HG)and PU(2PAN–HG) coating surfaces.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Fig. 9 Equivalent electrical circuit used for EIS fitting.
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PU(1PAN–HG) and PU(2PAN–HG) have higher RC and aremorestable than PU(HG) because of the anti-corrosion mechanism ofPAN. The RC of PU(1PAN–HG) and PU(2PAN–HG) are not much
Table 2 EIS fitting results from Nyquist and Bode plots of PU(HG), PU(1Ptimes
Sample Time RS, U cm2
CPEcoat
RQC, F sn$cm�2 n
HG 1 day 120.24 1.22 � 10�9 0.897 35 days 195.64 4.36 � 10�9 0.798 310 days 228.81 1.84 � 10�8 0.770 130 days 220.74 2.06 � 10�9 0.841 0
1PAN–HG 1 day 165.05 40.0 � 10�9 0.928 15 days 185.88 1.23 � 10�9 0.810 110 days 204.77 4.10 � 10�9 0.725 730 days 200.19 1.43 � 10�9 0.896 4
2PAN–HG 1 day 163.45 2.35 � 10�9 0.825 15 days 177.70 3.62 � 10�9 0.750 810 days 199.73 2.51 � 10�9 0.790 430 days 201.51 1.18 � 10�9 0.888 0
Fig. 10 Nyquist plots and Bode plots of PU(HG) (a and d), PU(1PAN–HG) (different times.
© 2021 The Author(s). Published by the Royal Society of Chemistry
different aer 1 day of immersion. The RC of PU(1PAN–HG) hasdecreased in turn from 1.15 � 107, 1.08 � 106, 7.98 � 105, and4.61 � 104, corresponding to 1, 5, 10, and 30 days of immersion.For the PU(2PAN–HG), the RC has slightly decreased in turn from1.61 � 107, 8.85 � 106, 4.62 � 106, and 0.17 � 106 U cm2, corre-sponding to 1, 5, 10 and 20 days of immersion.
All the Nyquist and Bode plots show one-time constantbehavior, revealing that only charger transfer resistance is presentin the electrochemical cell. In the Bode plots, the values of Z0 andZ00 represent the real and imaginary components of impedance inthe correlated relationship of jZj ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiZ 0 2 þ Z 00 2
p. Moreover, the
value of jZj determined from the Bode plots at low frequency (0.01Hz) can be regarded as a reference to assess the protection
AN–HA) and PU(2PAN–HG) immersed in 3.5% NaCl solution in various
C, U cm2
CPEdl
RCT, U cm2 s, U s�1/2QC, F sn$cm�2 n
.50 � 106 1.34 � 10�8 0.352 12.75 37.38
.96 � 106 2.26 � 10�8 0.279 1.37 � 105 1.09 � 105
.91 � 103 1.42 � 10�3 0.878 32.34 0.92 � 103
.40 � 103 1.21 � 10�7 0.407 1.29 � 103 0.33 � 103
.15 � 107 1.59 � 10�5 0.631 1.39 � 107 58
.08 � 106 1.87 � 10�11 0.464 7.97 � 104 6.93 � 104
.98 � 105 1.77 � 10�7 0.510 5.78 � 105 3.00 � 104
.61 � 104 5.68 � 10�6 0.409 4.63 � 105 19.98
.61 � 107 5.22 � 10�4 0.304 1.28 � 106 2.80
.85 � 106 1.63 � 10�7 0.715 1.36 � 103 0.11 � 103
.62 � 106 1.06 � 10�7 0.530 4.92 �106 50
.17 � 106 2.57 � 10�7 0.40 2.51 � 103 2.68 � 103
b and e) and PU(2PAN–HG) (c and f) immersed in 3.5% NaCl solution for
RSC Adv., 2021, 11, 31572–31582 | 31577
Fig. 11 jZjf¼0.01 Hz values of PU(HG), PU(1PAN–HG) and PU(2PAN–HG)immersed in 3.5% NaCl solution in different times.
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performance of the coatings. Fig. 11 shows the trend of the jZjf¼0.01
Hz values of PU(HG), PU(1PAN–HG), and PU(2PAN–HG). jZjf¼0.01 Hz
of PU(HG) has increased from 5.43 � 106 U cm2 to 11.70 � 106 Ucm2 aer immersion in 3.5% NaCl solution for 5 days due to thefeature of HG. Then, this value has decreased to 0.35� 106 U cm2
and 3.45 � 103 U cm2 aer 10 and 30 days, respectively. ThejZjf¼0.01 Hz values of PU(1PAN–HG) and PU(2PAN–HG) areenhanced compared with PU(HG). Aer 1 day immersion, jZjf¼0.01
Hz is 2.07� 107 U cm2 for PU(1PAN–HG) and 1.80� 107 U cm2 forPU(2PAN–HG). Aer 30 days immersion, these values havedecreased to 2.03� 104 U cm2 for PU(1PAN–HG) and 3.87� 104 Ucm2 for PU(2PAN–HG).
The results show that PAN played an important role inenhancing the corrosion resistance of the PU coating based on thecomplicated protection mechanism. That is the combination ofthe physical barrier mechanism, controlled inhibitor releasemechanism,43,47,71 cathodic protection mechanism, and the mostimportant is the anodic protection mechanism. For the anodicprotectionmechanism, reactions eqn (2) and eqn (3) show that thereduction of PAN happened in the passivation of the metalsubstrate, whereas the oxidized form of PAN can be recovered by
Fig. 12 Corrosion protection mechanism of GO, HG and PAN–HG.
31578 | RSC Adv., 2021, 11, 31572–31582
1/nM + 1/mPAN-ESm+ + y/nH2O / 1/nM(OH)y(n�y)+
+ 1/mPAN-LEB0 + y/nH+ (2)
m/4O2 + m/2H2O + PAN-LEB0 / PAN-EBm+ + mOH� (3)
The reduction of oxygen to hydroxide ions shis from themetal surface to the PAN electrolyte interface, associatedperhaps with reaction eqn (4):
m/4O2 + m/2H2O + me� / mOH� (4)
Corrosion protective mechanism
From the above observations and analysis, the anticorrosionmechanisms of the PU coating containing HG, 1PAN–HG, and2PAN–HG are proposed in Fig. 12. It can be discussed fromdifferent aspects as mentioned below.
First, either GO or PAN has a protective barrier mechanism,which can extend the diffusion path of the electrolyte andreduce the e� diffusion rate from the environment to the steelsubstrate. Meanwhile, GO has a large and exible sheet struc-ture that produces the best physical protective barrier, thebarrier mechanism of PAN is more complicated. The presenceof solid micro/nanollers in the organic coating exhibits bettercorrosion protection due primarily to:
- Stability of the redox activity of PAN and diminution of itsdegradation as the reaction eqn (5);
(5)
- The uniform distribution of PAN and the increased possi-bility of forming uniform passive layers on the metal surface.
- The restriction of penetration and lengthening of thediffusion path of water and oxygen molecules through thenanocomposite coating.
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- The ion-exchangeability of both HG and PAN can help totrap the anion Cl� and control the corrosion rate.
- The ability of trapping e� in the structure of HG that helpsto reduce the corrosion rate.
- The enhancement of mechanical stability of the coatingand its high adhesion strength to the metal substrate.
In addition, the corrosion protection of PAN–HG is mostlybecause of the anodic protection mechanism of PAN. ThePU(PAN–HG) coating protects the steel due to both barrier andblocking ability of the nanocomposite. That reaction is theconversion reaction of PAN when receiving e� and transformsfrom the ES to LS state. The PAN–LS layer at the contact surfaceof the paint-substrate will convert from the LS to ESstate.47,49,50,67,68,70 So that, aer forming the surface passivationlayer, electrons are continuously transferred to the externalenvironment, thereby inhibiting the corrosion reactionoccurring.72
Moreover, HG and PAN–HG can control the corrosion reac-tion rate due to the ion-exchangeability of both PAN and HG.Therefore, Cl� from the diffusion of the electrolyte and e�
generated from the corrosion reaction on the metal surface canbe trapped in the structure of HG and especially PAN, wherea redox reaction of PAN will occur and cause the anodicprotection mechanism.
ExperimentalMaterials
All chemicals used in this research, such as graphite, KMnO4,Zn(NO3)2$6H2O, Al(NO3)3$9H2O, ammonium persulphate (APS),aniline, and tartaric acid (TA) were synthesis grade andpurchased fromMERCK. For the preparation of the polyurethanecoatings, polyacrylate as the base and polyisocyanate as thehardener were obtained from Nippon Polyurethane Industry.
Synthesis composite from polyaniline with HG hybrid (PAN–HG). Graphene oxide (GO) was prepared using the Hummers'method, as previously described.75 The hybrid from GO andhydrotalcite was synthesized via coprecipitation of a mixture ofzinc nitrate and aluminum nitrate in the presence of GO and waslabeled HG. In detail, 6.8 g of Zn(NO3)2$6H2O and 13.6 g ofAl(NO3)3$9H2O were dissolved in 50 ml deionized water. 0.5 g ofGOwas dispersed in 100ml deionized water, and the GO solutionwas sonicated at 400 kHz for about 1 hour and then vigorouslystirred magnetically for about 30 minutes. The mixture of zinc–aluminum salt was dropped into the GO solution for co-precipitation. The pH of co-precipitation was controlled ata value of 9.5–10 using 0.2 M NaOH. The suspension wascentrifuged andwashed with distilled water until neutralized. HGwas then vacuum dried at 50 �C for 6 hours.
50 mg of HG was dispersed in 50ml of ethanol and sonicatedfor 2 hours, then vigorously stirred magnetically for about 30minutes in an ice bath. Aniline was dissolved in 25 ml of 0.5 MTA solution and APS was dissolved in the remaining 25 ml of0.5 M TA solution. The monomer solution was added dropwiseinto HG suspension for absorption. Then, the APS solution wasadded dropwise into the reaction for the polymerization ofaniline. The mixture changed from light yellow to blue, and
© 2021 The Author(s). Published by the Royal Society of Chemistry
nally turned to a dark green solution. The polymerization waskept in an ice bath at 2–4 �C for 12 hours. Then, the compositeparticles were washed with a mixture of ethanol : deionizedwater (50 : 50). The product was then dried in vacuum at 60 �Cfor 6 hours.
Preparation of PU coating containing composite PAN–HG.HG, 1PAN–HG, and 2PAN–HG were ground well and thendispersed in PU resin at 0.5 wt% mass loading (0.05 g) usingsonication, and then ultraspeed homogenized at 13 000 rpm.Then, the hardener was added to resin mixtures under high-speed stirring. The coatings were applied on CT3 steel plates,which were surface pre-treated well and dried at room temper-ature for 48 hours. The thickness of the coating was 10 � 0.02mm.
Characterization of the composites. The Fourier transforminfrared (FT-IR) spectra of the HG hybrid, 1PAN–HG, and 2PAN–HG composites were obtained using the KBr method ona Bruker Tensor 27 spectrometer, operated in the 400–4000 cm�1 region at 2 cm�1 resolution and for 32 scans.
Powder X-ray diffraction patterns of the HG hybrid, 1PAN–HG, and 2PAN–HG composites were obtained using a Bruker D8Advance diffractometer with Cu Ka radiation (1.5406 A) at roomtemperature under atmospheric pressure. Data were collectedin the 2 theta range of 5–70� with a step size of 0.02� anda scanning rate of 1� min�1.
The Raman spectra of GO and HG were recorded by LabSPEC fromHORIBA with the Raman shi range of 0–4000 cm�1,and using a laser at 532 nm.
The HG hybrid, 1PAN–HG, and 2PAN–HG composites andthe coating structure at the cross-sections of PU(HG), PU(1PAN–HG), and PU(2PAN–HG) were observed by scanning electronmicroscopy (SEM). SEM observations were carried out usinga Thermo Scientic E-SEM instrument.
Electrochemical characterization. The electrochemicalmeasurements were monitored with an AUTO LAB, PGS-statpotentiostat by using a three-electrode cell. The potentiody-namic polarization and electrochemical impedance spectros-copy (EIS) curves were measured in 3.5% NaCl solution at roomtemperature. For both measurements, an (Ag/AgCl, saturatedKCl) electrode was used as the reference electrode (Ag/AgCl,saturated KCl), and a platinum mesh panel of 2.0 cm �8.0 cm was used as the counter electrode. Before testing, theworking electrode was maintained at its open circuit potential(OCP) for 15 minutes until the OCP reached a steady state.
The potentiodynamic polarization measurement wascomposed of CT3 steel with an exposed area of 0.5024 cm2 asthe working electrode. For evaluation of the corrosion inhibi-tion effect, an accurate amount of HG hybrid, 1PAN–HG, and2PAN–HG was dispersed in 50 ml of 3.5% NaCl as the electro-lyte. The working electrode was immersed in the electrolytewith/without containing materials for 2 hours. The corrosioninhibition effect was studied by using potentiodynamic polari-zation curves with a 0.5 mV of potential scan rate, and startedfrom a potential of �0.03 V to 0.03 vs. OCP.
For the EIS measurement, the various PU coatings contain-ing HG, 1PAN–HG, and 2PAN–HG fabricated on the CT3 steelsurface were used as working electrodes with an exposed area of
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5.723 cm2. The frequency scanning was 100 kHz to 10 mHz withan amplitude of 10 mV.
Then, all potentiodynamic polarization curves and electricalimpedance spectra were calculated using Tafel tting, Nyquist,and Bode tting by EC-LAB version 10.36 soware. The corro-sion protection performance was determined by the corrosioncurrent density (icorr) and corrosion potential (Ecorr), and theprotection efficiency (h) was calculated by eqn (6) below.
h ð%Þ ¼ icorrðbareÞ � icorrðcpÞicorrðbareÞ � 100 (6)
where icorr(bare) and icorr(cp) are corrosion current densities ofthe CT3 steel electrode in 3.5% NaCl solution without/withcomposite particles.
Conclusions
In this research, composite PAN–HG was completely preparedusing directed polymerization of polyaniline on HG. The resultsindicated the layer structure of HG is still maintained andattached to the PAN crystal surface with a strong bond betweenthose particles. The electrochemical properties of thesecomposites were evaluated by potential polarization curves.Then, the results were calculated with Tafel tting to give theeffective corrosion protection at 73.11%, 88.46%, and 95.49%,corresponding to HG, 1PAN–HG, and 2PAN–HG, respectively.The PU coating samples was prepared with 1% content of HG,1PAN–HG, and 2PAN–HG. The corrosion protection abilities ofthe coating samples were evaluated using the EIS method. Asa result, the PU(2PAN–HG) coating shows the most effectiveprotection of the CT3 steel substrate with the RC of the coatingaer 1 day immersed in 3.5% NaCl solution at about 1.61 � 107
U cm2. Aer 30 days of immersion, the RC value was 0.17 � 106
U cm2. In addition, the mechanism of the corrosion protectionwas explained as the combination of both barrier mechanism ofHG and PAN, ion-exchange mechanism to trap the Cl� andcontrol the corrosion rate, the ability to trap e� in the structureof HG and the most noticeable is anodic protection mechanismcoming from PAN. This helps to make the enhancement of thecorrosion protection ability of PAN–HG compared with HG.
Author contributions
Boi An Tran: Corresponding author, Conceptualization, Inves-tigation, Writing-Original Dra, Writing-Review & Editing.Huynh Thanh Linh Duong: Investigating, Editing. Thanh ThaoPhan: Science Advisor, Writing-Review. Thi Xuan Hang To:Science Advisor, Writing-Review.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors would like to thank Dr Thanh Thao Phan and DrThi Xuan Hang To for their science and technical assistance,
31580 | RSC Adv., 2021, 11, 31572–31582
and the Institute of Chemical Technology for their analysisequipment support.
Notes and references
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